Travel agent took payment now wants more money or canceling flights?

I have an issue whereby I had flights booked with Delta through a travel agent flying from Sydney Australia to the US and back again. My situation changed whereby I needed to alter my flights and fly from Ireland to the US instead.

Having originally been told a change was was not possible, I was then informed that I could have the tickets reissued and pay the differences in fare and tax on top of the $200 per person change fee. The travel agent was twice told this by Delta.

The travel agent sent me through a quote with the updated flight itinerary and I paid the travel agent $493 to change the flights.

I also altered other flights I had booked separately based on this updated quote and payment.

I have now received information from the travel agent saying they received incorrect information from Delta and that it would have to be recalculated with the fare being of equal or greater value plus the difference in the taxes. They are now looking for me to pay an extra $600.

I received the following email yesterday if I didn't wish to pay the $600.

If you choose not to take this offer Im happy to refund you the money you paid for the change but you will be left without any flights and a credit that can only be used ex Australia which you are more then welcome to argue with Delta about, I have done all I can on my end and got quite a good solution considering you were going to loose the entire thing.

Can they do this?

Surely they should honor what was paid for originally?

Any information on similar matters people could provide would be great.